The official says the U.S. has no evidence that either side used chemical weapons in the attack in northern Syria.

The official says the origin of the attack is still unclear.

But the official says the Organization for the Prohibition of Chemical Weapons, too, is reporting no independent information of chemical weapons use.

The official wasn’t authorized to speak publicly on the matter and spoke on condition of anonymity.

The rebels deny the Syrian government claim and blame regime forces for Tuesday’s missile attack on the Khan al-Assad village.

But Russia’s Foreign Ministry is supporting the assertion by Assad’s government, calling rebel use of an unidentified chemical agent “extremely dangerous.”

The ministry said the attack represented an “extremely dangerous” development in the two-year Syrian crisis. It said the rebels detonated a munition containing an unidentified chemical agent early Tuesday in the province of Aleppo. It didn’t give further details.

Syria’s state-run news agency has also accused the rebels of the attack, saying 25 people died, but the rebels say government forces fired the weapon.

Russia said that the evidence of rebels getting hold of weapons of mass destruction had taken the confrontation to a new stage.

Russia has been the main ally of Assad’s regime since the start of the uprising, in which more than 70,000 have died.